Introduction: The Roman empire stretched on one diagonal from Hadrian’s Wall to northern Iraq, and, on the other, from the mouth of the Rhine to the Atlas mountains of north Africa. It was the largest state that western Eurasia has ever seen: larger, for instance, than even the enlarged European Union, and certainly dwarfed the largest of Europe’s medieval empires. It was also extremely long-lived. Transylvania and Britain apart, where it lasted only one hundred and seventy years and three hundred and fifty years respectively, Roman rule prevailed over the remainder of its domains for an astonishing five hundred years. And all this in a period where the speed of bureaucratic functioning and of military response rattled along at 45 kilometres a day, something like one tenth of modern counterparts. Measured in terms of how long it took real people to get places, the Roman empire was arguably ten times as big, even, than it appears on the map.

The epic scale of its existence has always sharpened interest in the causes of collapse. And, since Gibbon, while a role has always been allocated to outside invasion, explanation has tended to focus on a range of internal transformations and problems as the prime movers in the processes of Roman imperial collapse which brought the western half of it to an end in the fifth century AD. Gibbon famously drew attention to the empire’s top-heavy ‘stupendous fabric’, and to the supposedly problematic effects of Christianisation, while, by the mid-twentieth century, attention commonly concentrated upon the economic collapse and political chaos which were seen as typical of the later Roman empire of the fourth century.